The influence of body position and differential ventilation on lung dimensions and atelectasis formation in anaesthetized man.

The effects of body position and anaesthesia with mechanical ventilation on thoracic dimensions and atelectasis formation were studied by means of computerized tomography in 14 patients. Induction of anaesthesia in the supine position reduced the cross-sectional area for both lungs and caused atelectasis formation in dependent lung regions in 4/5 patients. Conventional ventilation with positive end-expiratory pressure (PEEP) increased thoracic dimensions and reduced, but did not eliminate, the atelectatic areas. The vertical diameters of both lungs were smaller in the lateral position as compared to the supine position (16.7 vs 10.4 cm in the left lung and 17.3 vs 12.8 cm in the right lung). The lateral positioning also caused a large reduction of the atelectatic area in the non-dependent lung. Differential ventilation with selective PEEP to the dependent lung eliminated (3/8 patients) or reduced (5/8 patients) dependent lung atelectasis. It can be concluded that lung geometry is altered in the lateral position: the shape of the lung makes the vertical diameter of each lung less in the lateral position, compared to the supine position. The atelectatic areas are mainly located in the dependent lung in the lateral position, and these atelectatic areas could be further reduced by selective PEEP to this lung.